Semiconductor laser diodes have a high electrical-to-optical conversion efficiency, and can presently achieve optical power levels of a few Watts or even tens of Watts per single emitter laser diode, and tens to hundreds of Watts when manufactured as laser arrays. Due to high efficiency, reasonable power levels, and high spectral and directional brightness, laser diodes find applications in many areas, such as material processing, offset printing, medical treatment, pumping of solid state lasers, and pumping of fiber lasers.
Still, some applications require optical powers even greater than those that can be obtained from a single laser emitter. Many applications further require that the output light from a laser source be provided as a single optical beam, for example for coupling into an optical fiber, which generally has a substantially circular or polygonal cross-section and has a substantially symmetrical acceptance angle. To obtain the highest brightness out of the fiber, light beams from multiple single emitter diode lasers are coupled into a single fiber either intermixed, for example using a multi-mode beam combiner as disclosed for example in U.S. Pat. No. 7,212,554 that is incorporated herein by reference, or stacked in their fast axis direction, as described for example in U.S. Pat. Nos. 6,898,222, 7,668,214, 7,733,932, 7,773,655, and 8,427,749, all of which are incorporated herein by reference. For example, an array of 3-10 individual laser emitters with a 100 micrometers (um) aperture width in the slow axis can be coupled into a fiber with a 105 um diameter and 0.15 NA (numerical aperture) by stacking individual laser beams in the fast axis direction.
There is however a limit how many laser beam can be stacked for efficiently coupling into a fiber. Since the diode laser emission is typically polarized, polarization beam combining may be used to couple light emitted by two arrays of single emitters into a single fiber, thereby doubling the power and brightness of the output beam. Examples of such laser beam combining devices, which incorporates both the spatial stacking of equally polarized laser beams with polarization multiplexing of stacked beams from two laser arrays, are disclosed in U.S. Pat. No. 4,978,197 and U.S. Pat. No. 8,427,749, which is incorporated herein by reference.
Some application further require that the output laser beam is wavelength-stabilized, to prevent the optical spectrum of the beam to wonder with changes in injection current, operating temperature, or due to uncontrolled back reflections. One conventional way to achieve such wavelength stabilization is to use an external reflector with a narrow reflection band, to provide a wavelength-selective optical feedback to the laser at a level that is suitable to effectively lock the laser spectrum to the narrow reflection band of the external reflector. For example, U.S. Pat. No. 6,041,072 discloses an arrangement where output light from multiple laser diodes emitting at different wavelengths is coupled into a single-mode optical fiber using wavelength multiplexers, and the lasers are individually stabilized using multiple fiber Bragg gratings (FBGs) formed in the fiber. U.S. Pat. No. 7,212,554 discloses using a volume Bragg grating (VBG) to stabilize multiple lasers which output beams are first intermixed and combined in a single beam using a multimode combiner. U.S. Pat. No. 8,427,749 discloses using a VBG in the optical path of vertically-stacked beams from a plurality of laser diodes.
One drawback of using external reflectors for wavelength stabilization is that returning a portion of the laser light back into the laser cavity reduces the useful output power from the light source. This drawback worsens when the lasers are to be stabilized over a wide range of operating parameters and laser characteristics that affect the spectral position of the optical gain peak in the laser, necessitating higher reflectivity of the grating to affect a sufficiently broad range of wavelength locking.
Accordingly, a need exists for providing high-power wavelength-stabilized laser modules and methods for providing high-brightness optical beams that obviate at least some of the disadvantages of the prior art.